In 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), OFDMA (Orthogonal Frequency Division Multiple Access) is adopted as a communication scheme for a downlink from a base station (also referred to as an eNB) to a terminal (also referred to as a UE (User Equipment)) is adopted, and SC-FDMA (Single Carrier-Frequency Division Multiple Access) is adopted as a communication scheme for an uplink from a terminal to a base station (see, for example, NPLs 1 to 3).
In LTE, a base station performs communication by assigning an RB (Resource Block) in a system band to a terminal on a time unit basis, the time unit being called a subframe. FIG. 1 shows a configuration example of a subframe of an LTE uplink shared channel (PUSCH: Physical Uplink Shared Channel). As shown in FIG. 1, one subframe is made up of two time slots. In each slot, a plurality of SC-FDMA data symbols and DMRS's (Demodulation Reference Signals) are time-multiplexed. Upon receipt of a PUSCH, the base station performs channel estimation using the DMRS's. After that, the base station performs demodulation/decoding of the SC-FDMA data symbols using a result of the channel estimation.
Further, in LTE, HARQ (Hybrid Automatic Repeat Request) is applied to downlink data. In other words, a terminal feeds back a response signal indicating an error detection result of downlink data to a base station. The terminal performs CRC (Cyclic Redundancy Check) for the downlink data, and feeds back an acknowledgement (ACK) if there is not an error in an operation result of the CRC and a negative acknowledgement (NACK) if there is an error in the operation result of the CRC, to the base station as the response signal. An uplink control channel such as a PUCCH (Physical Uplink Control Channel) is used to feed back this response signal (that is, the ACK/NACK signal).
A plurality of formats are selectively used according to situations of the terminal transmitting the ACK/NACK signal through the PUCCH. For example, if there is not control information to be transmitted other than the ACK/NACK signal and an uplink scheduling request, a PUCCH format 1a/1b is used. On the other hand, if transmission of the ACK/NACK signal overlaps with feedback of CSI (Channel State Information) which is periodically transmitted through an uplink channel, a PUCCH format 2a/2b is used.
As shown in FIG. 2, each of a plurality of ACK/NACK signals transmitted from a plurality of terminals in the PUCCH format 1a/1b is spread by a ZAC (Zero Auto-Correlation) sequence having a Zero Auto-correlation characteristic (multiplied by the ZAC sequence) on a time axis and is code-multiplexed in the PUCCH. In FIG. 2, (W(0), W(1), W(2), W(3)) indicates a Walsh sequence with a sequence length of 4, and (F(0), F(1), F(2)) indicates a DFT (Discrete Fourier Transform) sequence with a sequence length of 3.
As shown in FIG. 2, first in a terminal, the ACK/NACK signal is primarily spread to frequency components each of which corresponds to one SC-FDMA symbol by the ZAC sequence (with a sequence length of 12) on a frequency axis. In other words, the ZAC sequence with a sequence length of 12 is multiplied by ACK/NACK signal components each of which is represented by a complex number. Next, each of the primarily spread ACK/NACK signal and the ZAC sequence as a reference signal are secondarily spread by the Walsh sequence (with a sequence length of 4; W(0) to W(3)) and the DFT sequence (with a sequence length of 3; F(0) to F(2)), respectively. In other words, components of the signal with a sequence length of 12 (the primarily spread ACK/NACK signal or the ZAC sequence as a reference signal) are multiplied by components of an orthogonal cover code (OCC) sequence (the Walsh sequence or the DFT sequence), respectively. Furthermore, the secondarily spread signals are converted to signals with a sequence length of 12 on the time axis by IDFT (Inverse Discrete Fourier Transform) or IFFT (Inverse Fast Fourier Transform). Then, a CP (Cyclic Prefix) is added to each of the signals after the IFFT, and a 1-slot signal composed of seven SC-FDMA symbols is formed.
Further, as shown in FIG. 3, a PUCCH is assigned to each terminal in subframes.
ACK/NACK signals from different terminals are spread (multiplexed) with ZAC sequences defined by different cyclic shift indexes or orthogonal cover code sequences corresponding to different sequence numbers (orthogonal cover (OC) indexes). An orthogonal cover code sequence is a set of a Walsh sequence and a DFT sequence. Further, the orthogonal cover code sequence may be referred to as a block-wise spreading code sequence. Therefore, a base station can separate the plurality of code-multiplexed ACK/NACK signals by using conventional despreading and correlation processing (see, for example, NPL 4).
By the way, recently, M2M (Machine-to-Machine) communication is promising which realizes services by autonomous communication among pieces of equipment without judgment of users as a structure supporting a future information society. A specific application example of an M2M system includes a smart grid. The smart grid is an infrastructure system for efficiently supplying lifelines such as electricity and gas. For example, the smart grid performs M2M communication between a smartmeter disposed in each home or building and a central server to autonomously and efficiently adjust demand balance of resources. Other application examples of the M2M communication system include a monitoring system for article management, environmental sensing or telemedicine, remote management of stock or charging for vending machines, and the like.
As for the M2M communication system, attention has been paid especially to utilization of a cellular system having an extensive communication area. In 3GPP, standardization of enhancement of a cellular network for M2M called MTC (Machine Type Communication) has been promoted (see, for example, NPL 5) in standardization of LTE and LTE-Advanced, and examination of specifications has been started, with cost reduction, power consumption reduction and coverage enhancement as requirements. Especially, in the case of terminals, such as smartmeters, which are virtually immobile unlike handset terminals which are often used by users while the users are moving, it is necessary to secure coverage to provide services. Therefore, in order to support a case where, at a place in an existing LTE and LTE-Advanced communication area where an LTE or LTE-Advanced terminal cannot be used, such as the underground of a building, a terminal (an MTC terminal) usable at such a place is disposed, “coverage enhancement (MTC coverage enhancement)” to further expand a communication area has been examined.